This is the massage technique I use that is highly effective to break up a stubborn crop mass. You first need to give two teaspoons of oil. I use coconut oil slightly chilled for ease in getting it into the chicken's beak. Then use the technique described by @ChicNmom concentrating on one spot in the center of the mass. Focus. Keep massaging that one point until you feel it break up.**I used a massage technique called frictioning. I'm a massage therapist and it is just like it sounds. Get the fingers center mass . Dig in a little and make quick short back and forth motions. Maybe 20 to 30 seconds at a time.
If it refuses to break up, give more oil, and keep trying. One time I had such a stubborn case that it took an entire hour of constant massage and repeated doses of oil to get it to break up. When it began to break up, it seemed to fall apart all at once and I kept massaging until the crop was emptied of this solid material. This patient also had sour crop due to the mass fermenting. So I followed up with miconazole for seven days.